This article presents a brief overview of the impact that
antibiotic use in clinical medicine and in other settings, such as
agriculture and animal farming, exerts on antimicrobial resistance.
Resistance has been described to all antibiotics that are currently in
use, and resistant strains were sometimes reported as soon as months
after specific antibiotics became commercially available. There are many
examples in which the increasing prevalence of resistant microbial
strains jeopardized the continuing effective use of the respective
antibiotics in clinical medicine. In addition to resistant infections
that occur in health-care establishments, one of the recent challenges
is the emergence of pathogens, such as MRSA, in the community, among
individuals without any apparent risk factors for the infection. The
transmission of resistant pathogens and antimicrobial resistance
determinants across different components of the ecosystem transforms
antibiotic resistance into a topic that extends beyond the scope of
clinical medicine and needs to be visualized through an integrated
global perspective that should incorporate a broad range of disciplines,
including molecular genetics, microbiology, food science, ecology,
agriculture, and environmental science. Understanding this complex
multi- and interdisciplinary framework will enable the implementation of
the most appropriate interventions toward determining the dynamics of
antimicrobial resistance, limiting the emergence and spread of resistant
strains, and ensuring the ongoing effective and safe use of antibiotics.

Infectious diseases were among the leading causes of morbidity and
mortality in the 1900s, and tuberculosis and bacterial pneumonia were
some of the most frequent causes of death during childhood (Cohen,
1998). In 18th-century France, half of all children did not reach age 2;
infant mortality in Bombay was ~50% between 1900 and 1920; and during
the second half of the 19th century, up to one-third of the children in
Western Europe and the United States did not reach their first birthday,
with half of those deaths being caused by infectious diseases (Lee et
al., 2007; Mulholland, 2007). Kohn and Weiner (1936) examined 1000
children admitted with non-tuberculous pneumonia to the pediatric
services of Mount Sinai Hospital in New York between November 1926 and
March 1933; they found overall mortality rates of around 20% and even
higher rates, around 39%, among children younger than 1.

The discovery of penicillin marked one of the defining moments of
the 20th century. In 1928, while studying the morphology of
Staphylococcus aureus strains and preparing to leave for vacation, Louis
Pasteur left several plates on a bench in his lab, and upon return, he
discarded a few of them, which became contaminated with mold. As he was
explaining his experiments to a colleague, he retrieved one of the
discarded plates, which contained mold. Pasteur noticed that bacterial
colonies on that plate had a different morphology and were undergoing
lysis (Bentley, 2005; Davies & Davies, 2010; Kong et al., 2010).
This observation marked one of the most important discoveries in the
history of medicine and led to the development of penicillin.

O The Emergence of Resistant Bacteria

In 1943, [beta]-lactam antibiotics were introduced. Soon after, the
first resistant strains--which had acquired the ability to hydrolyze the
p-lactam ring of the antibiotic by using enzymes known as
[beta]-lactamases-threatened to compromise the effectiveness of this
group of antibiotics (Abraham & Chain, 1940; Llarrull et al., 2010).
The percentage of S. aureus strains that are resistant to penicillin
increased from 6% in 1946 to 50% by 1950 and to 80-90% in recent years
(Livermore, 2000; Deurenberg & Stobberingh, 2008). But the ability
of some bacterial strains to hydrolyze and inactivate the antibiotic was
reported in 1940, several years before penicillin became commercially
available, which has prompted the question of which emerged first--the
antibiotic or the resistance (Abraham & Chain, 1940; Davies &
Davies, 2010). This question becomes particularly intriguing in light of
a recent report describing [beta]-lactamases in remote Alaskan soils
that lack known antibiotic exposure and, except for researchers, were
not visited by others (Allen et al., 2009).

The 1960 introduction of methicillin, a semisynthetic penicillin
derivative, overcame the problem of penicillin resistance, but bacterial
strains resistant to methicillin emerged rapidly, by a new mechanism:
they acquired a new gene, encoding a protein with a lower ability to
bind the antibiotic, but capable of continuing bacterial cell-wall
synthesis even in the presence of the antibiotic (Llarrull et al.,
2010). The first methicillin-resistant strains were reported in 1961,
less than a year after the antibiotic was introduced commercially
(Jevons, 1961; Palumbi, 2001; Lowy, 2003), and, in some countries,
40-50% of all S. aureus isolates are currently resistant (Lowy, 2003;
Grundmann et al., 2006). Between 1999 and 2005, the number of admissions
for MRSA infection in U.S. hospitals more than doubled, from
approximately 127,000 to 278,000 (Klein et al., 2007).

Recently, Rice (2008) identified a group of pathogens that he
refers to as "ESKAPE," which includes Enterococcus faecium,
Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii,
Pseudomonas aeruginosa, and Enterobacter species. These are pathogens
that, through their ability to escape susceptibility to antimicrobials,
have become public health concerns in both the developing and the
developed worlds. As Rice emphasizes, these pathogens stand out not only
for the vast number of nosocomial infections that they cause worldwide,
but also for their ability to serve as models for understanding
bacterial pathogenesis, transmission, and antimicrobial resistance in
other pathogens (Rice, 2008).

Hawkey (1998) describes four major mechanisms to explain how a
bacterial cell may become resistant to an antibiotic. Antibiotic
modification, the most frequent one, occurs when a resistant bacterium
modifies and inactivates the antibiotic, which does not affect its
cellular target any longer. The enzyme [beta]-lactamase, which cleaves
the [beta]-lactam ring of the antibiotic, provides a relevant example.
The second mechanism occurs when a bacterium prevents the antibiotic
from entering the cell or removes the antibiotic from the cell by efflux
pumps, and is illustrated by the tetracycline efflux system that was
first described in Escherichia coli. Efflux pumps were subsequently
reported for different antibiotics in other bacteria (McMurry et al.,
1980; Poole, 2005). The third mechanism involves altering the antibiotic
target site: the antibiotic is able to enter the cell and to reach its
target, but the target is not affected any longer because it underwent
structural changes. For example, a recently described fluroquinolone
acetyl transferase can acetylate and reduce the activity of certain
fluoroquinolones, conferring resistance (Robicsek et al., 2005). The
fourth mechanism, called "bypass," occurs when the bacterium
retains the sensitive target but makes an alternative, additional target
that is able to bind the antibiotic and confer resistance. The best
example is provided by some S. aureus strains that acquire a gene that
encodes a new penicillin-binding protein, PBP2a or PBP2'.
Penicillin binding proteins (PBPs) are a group of enzymes that insert
peptidoglycan precursors into the bacterial cell wall. Staphylococcus
aureus has four such enzymes: PBP1, 2, 3, and 4. Some strains that
produce an additional protein, PBP2a or PBP2', which is the product
of the mecA gene, acquire reduced affinity for methicillin and can
continue cell wall synthesis, despite inhibitory concentrations of the
antibiotic and even though the other PBPs are blocked by the antibiotic.
These are known as the methicillin-resistant S. aureus (MRSA) strains
that currently represent an emerging public health concern (Katayama et
al., 2000; Chambers, 2003; Guignard et al., 2005).

Bacteria have developed resistance to all known classes of
antibiotics discovered to date (Alanis, 2005), and often within years
after individual antibiotics were commercialized (Palumbi, 2001), in
many situations threatening the effective use of the respective
antibiotics. Resistance was initially observed in health-care settings,
but it soon emerged in the community, an even more worrisome trend that
was described worldwide for several microorganisms (Furuya & Lowy,
2006). In addition, some bacteria, also known as "multi-drug
resistant strains" or "superbugs," became resistant to
multiple antibiotics (Alanis, 2005). Infections with resistant pathogens
are associated with higher morbidity and mortality, increased costs, and
longer hospital stays (Cosgrove, 2006; Heymann, 2006). For example,
mortality from MRSA is approximately double that from MSSA, its
methicillin-sensitive counterpart (Gould, 2006).

Coupled with the decreasing number of new antibiotics that are
developed and approved, the increasing prevalence of antimicrobial
resistance raises well-founded concerns about whether we may already
have entered the post-antibiotic era (Alanis, 2005).

O MRSA in Hospitals & in the Community

By the end of the 1940s, half of the S. aureus strains in British
and U.S. hospitals were resistant to penicillin (Grundmann et al.,
2006). Methicillin, a new [beta]-lactam antibiotic that is not degraded
by p-lactamases, was commercially introduced in October 1960;
approximately 6 months later, the first methicillin-resistant strains
were already being described (Jevons, 1961). Since then, MRSA has become
the most frequently identified antibiotic-resistant pathogen in many
countries and has created a public health emergency of global concern
(Grundmann et al., 2006). It is estimated that among the ~2 billion
individuals worldwide who carry S. aureus, between 2 and 50 million
carry MRSA (Grudmann et al., 2006). In addition to the increased
mortality and morbidity associated with MRSA infections, some authors
found that MRSA strains do not replace MSSA strains but, instead, cause
additional disease (Gould, 2009).

MRSA infections were increasingly reported in health-care
facilities, and hospitalization, dialysis, catheters, and long-term
antibiotic use emerged as some of the main risk factors. In addition to
the hospital-acquired MRSA (HA-MRSA), a new phenomenon in recent years
is MRSA infections that occur in the community, in apparently healthy
individuals who lack the known risk factors for the infection, and this
has become known as "community-acquired" or
"community-associated" MRSA (CA-MRSA) (Namii et al., 2003).
After 1980, when CA-MRSA was first reported in the United States
(Saravolatz et al., 1982), this pathogen was increasingly described
throughout the country and worldwide, and in 2007 it was estimated to
represent the most frequent cause of skin and soft-tissue infections in
U.S. emergency departments (Klevens et al., 2007).

The Centers for Disease Control and Prevention propose that the
CA-MRSA definition be applied when the MRSA infection occurs in an
outpatient setting, or within 48 hours after hospital admission, in an
individual who lacks MRSA colonization or infection, hospitalization or
residence in a long-term health care facility, surgery, or dialysis in
his or her medical history (Nastaly et al., 2010). It is important to
note that this classification is not based merely on the place where the
infection occurs, as the name would imply, but HA-MRSA is different form
CA-MRSA in several respects, which include molecular and epidemiologic
characteristics, clinical manifestations, risk factors, risk groups, and
the antibiotic resistance profile (McCarthy et al., 2010; Nastaly et
al., 2010). CA-MRSA strains differ from HA-MRSA at the molecular level
and often harbor several virulence factors, such as the Panton-Valentine
leukocidin gene, which is responsible for producing cytotoxins
associated with tissue necrosis (Weber, 2005; Klevens et al., 2007;
Deurenberg & Stobberingh, 2008; Guilbeau & Broussard, 2010).
Although HA-MRSA strains are often resistant to multiple antibiotics,
CA-MRSA is often found to be susceptible (Huang et al., 2006; Nastaly et
al., 2010). HA-MRSA occurs more frequently in hospitalized patients, the
elderly, and health-care workers, whereas the CA-MRSA risk groups
include participants in competitive sports, athletes, soldiers,
prisoners, and drug users (CDC, 2003; Nastaly et al., 2010) and the
infection often affects young, healthy individuals (Naimi et al., 2003).

As HA-MRSA enters the community and CA-MRSA emerges in health-care
facilities, distinguishing these two strains is becoming increasingly
difficult (McCarthy et al., 2010). Some authors suggest that CA-MRSA
replaces HA-MRSA in hospital settings (Popovich et al., 2008), whereas
others have found that, instead, CA-MRSA adds to the existing HA-MRSA
burden (Klein et al., 2009). It is important to remember that the
worldwide incidence of MRSA is on the rise, its epidemiology is
changing, and this pathogen is an increasingly significant public health
concern in health-care facilities and in the community. In recent years,
MRSA was associated with ~19,000 deaths annually among hospital patients
in the United States, a figure that is higher than the combined number
of deaths caused by HIV/AIDS and tuberculosis (Boucher & Corey,
2008).

O The Spread of Pathogens & Resistance

The use of antimicrobial agents increasingly emerges as a selective
force that promotes the emergence of resistant microbial strains
(Lipsitch & Samore, 2002). A bacterial strain can become resistant
to antimicrobials in two major ways. The first mechanism involves the
introduction of new mutations into existing genes, such as a mutation in
a gene encoding an antibiotic target, which leads to the inability of
the antibiotic to bind its target. For example, resistance to quinolones
can be acquired by mutations in the genes encoding DNA topoisomerases
(Drlica & Zhao, 1997; Blazquez, 2003). In addition, it is important
to appreciate that drug resistance is mobile and that bacteria may
acquire new resistance genes from other bacteria by means of plasmids,
naked DNA, bacteriophages, or transposons (Levy & Marshall, 2004;
Boerlin & Reid-Smith, 2008). The acquisition of new resistance genes
can be intracellular, such as the transfer of a gene from a plasmid to
the chromosome, or intercellular, which is often called
"lateral" or "horizontal" gene transfer (Boerlin
& Reid-Smith, 2008). As a result of horizontal gene transfer,
antimicrobial resistance may be transferred between bacteria from the
same species or between different bacterial species (Barlow, 2009). This
process can be accomplished by different mechanisms, which include
transformation, in which bacteria take up DNA from the environment and
incorporate it into their genome; conjugation, which involves the
transfer of DNA by direct contact between a donor and a recipient
bacterium; and transduction, when bacterial viruses, also known as
bacteriophages, transfer genetic information between bacteria (Boerlin
& Reid-Smith, 2008; Juhas et al., 2009).

One of the most elegant examples to illustrate how antibiotic
resistance can spread from one species to another was provided by Levy
et al. (1976), who showed that in chickens fed with tetracycline,
antibiotic-resistant plasmids were transferred from chickens to chickens
and from chickens to humans. More recently, Brody et al. (2008)
performed a whole-genome analysis to compare several S. aureus strains
and found over 14 different DNA regions that were shared between a human
MRSA isolate that is endemic in hospitals and strains that cause bovine
mastitis, which indicates that horizontal gene transfer may have
occurred between various strains.

Bates et al. (1994) reported that vancomycin-resistant enterococci
are present in nonhuman sources, including farm animals, uncooked
chicken, and sewage. When the authors performed ribotyping for 42 of the
62 strains with high-level vancomycin resistance that they had isolated,
they distinguished 12 ribotypes, two of which, found in nonhuman sources
and in humans, were indistinguishable: a sewage isolate was identical to
a human blood and urine isolate, and a pig isolate was identical to a
human stool isolate. This study found, for the first time, clinical
isolates and strains identified from nonhuman environmental sources that
have indistinguishable ribotypes.

Resistant pathogens can be transmitted from animals to humans.
Manian (2003) reported that a diabetic patient developed recurrent MRSA
infection of the leg, and his wife had cellulitis. MRSA isolated from
the couple's dogs' nostrils had the same antibiotic resistance
pattern as the one cultured from the couple's nostrils and wounds,
and the strains showed indistinguishable patterns by PFGE typing. The
couple confirmed that the dog slept in the same bedroom, and often
licked their faces. Recurrences of the infection in the couple were
stopped only when the dog was also treated with antibiotics. This and
several other studies have revealed that resistant microorganisms can be
transmitted between animals and humans. Rwego et al. (2008) collected
fecal samples from humans, livestock, and mountain gorillas from the
Bwindi Impenetrable National Park in southwestern Uganda, between May
and August 2005. The authors reported that a particular group of
gorillas, which were the focus of tourism activities and ventured into
areas used by humans, harbored E. coli strains that showed genetic and
antibiotic resistance similarities with human and livestock bacterial
isolates, illustrating that habitat overlap could facilitate the
cross-species transmission of microbial pathogens.

An important emerging concept, relevant to the spread of resistant
microorganisms between different environments and species, is the idea
that the medical and scientific communities have mostly focused on
antimicrobial resistance in bacteria that are pathogenic, but that a
broader view is needed, to incorporate resistance genes from pathogenic
as well as nonpathogenic bacteria that together reflect the sum of all
the microbial genomes. The resistance genes found in all these
microorganisms, pathogenic and nonpathogenic, have been termed the
"resistome," a concept that promises to advance our
understanding of the global ecology of bacterial resistance
(D'Costa et al., 2006; Wright, 2007). To illustrate the large
number of resistant bacteria that exist in addition to the medically
relevant ones, D'Costa et al. (2006) tested 480 bacterial strains
isolated from soil samples for sensitivity to 21 different antibiotics,
and they reported that every strain was resistant to seven or eight
antibiotics, on average. In light of the wealth of antibiotic-resistant
bacteria that exist in nonclinical samples, and the established
mechanisms that allow pathogens and resistance determinants to be
transmitted between ecosystems, it is imperative to consider
antimicrobial resistance not simply as a clinical issue but as a global,
ecological concern.

O Antibiotics in Agriculture

In addition to antibiotic use in clinical medicine, huge amounts of
antibiotics are being used in agriculture, aquaculture, and animal
farming. These environments are in a dynamic state with one another and
also with other ecosystems, and resistant microorganisms or genetic
determinants of resistance can be transmitted between them.

In the early 1940s and 1950s, it was noticed that supplementing the
food of healthy animals with antibiotics contributes to their growth
rate (Hammerum et al., 2010). Soon, this became a worldwide practice,
and in addition to the antibiotics used in farm animals for therapeutic
purposes, to treat infections, large amounts of antibiotics started to
be used as growth promoters at subtherapeutic concentrations (van den
Bogaard et al., 2000; Nikaido, 2009). Approximately 48% of the
>10,000 tons of antibiotics used in the European Union and
Switzerland in 1997 was used in animals. From this amount, 33% was for
therapeutic and prophylactic purposes and 15% for growth promotion, with
large country-to-country variations (van den Bogaard & Stobberingh,
2000). Antibiotic additives as growth promoters were banned in the
European Union on 1 January 2006 (Castanon, 2007). In the United States,
between 50% and 80% of the antibiotics that are manufactured are being
used in agriculture (Lipsitch et al., 2002; Smith et al., 2002). The
Union of Concerned Scientists estimated that, annually, ~24.6 million
pounds of antimicrobials are given to animals in the absence of disease,
for nontherapeutic purposes (Mellon et al., 2001), and the Institute of
Medicine (1989) reported that over half of the ~31 million pounds of
antibiotics produced annually are being directed to farm animals. Teuber
(2001) estimated that approximately half of the >1 million tons of
antibiotics released into the biosphere during the past 50 years was for
veterinary and agricultural purposes.

O The Example of Avoparcin

In 1975, avoparcin was approved as a food additive and growth
promoter in many countries worldwide, including the European Union, but
not in Sweden, the United States, or Canada (van den Bogaard &
Stobberingh, 2000; Hammerum et al., 2010).

Cross-resistance between avoparcin and vancomycin occurs because
the same gene, vanA, carries the resistance determinant to both
antibiotics (Bager et al., 1999). In 1995, Aarestrup revealed that the
agricultural use of avoparcin could select for vancomycin-resistant
enterococcal strains in humans (Aarestrup, 1995). To compare
conventional poultry farms with ecological ones (which did not use
antibiotics therapeutically or as a feed additive) in Denmark, the
authors examined fresh fecal samples and detected vancomycin- and
avoparcin-resistant strains on five of eight conventional farms, but no
resistance was observed among strains isolated from ecological farms.

In The Netherlands, it was reported that approximately 1500 kg and
1260 kg of vancomycin were used for human therapy in 1996 and 1998,
respectively, and an estimated 80,000 kg of avoparcin was used annually
in farming until 1997 (van den Boraard et al., 2000). Avoparcin use was
discontinued in Denmark and Norway in 1995, in Germany in 1996, in the
rest of the European Union and Korea in 1997, and in New Zealand and
Taiwan in 2000 (Lauderdale et al., 2007; Hammerum et al., 2010). After
this ban, a reduction in vancomycin-resistant bacteria from animals and
humans in the community was recorded in many of these countries (Witte,
2000).

Aarestrup et al. (2001) found that between 1995 and 2000, the
proportion of vancomycin-resistant E. faecium isolates in Denmark
decreased from 72.7% to 5.8% in broilers, and from 20% to 6% in pigs.
Similar findings were reported from other countries, such as The
Netherlands, where van den Bogaard et al. (2000) revealed, in addition,
that within 2 years after the avoparcin ban, the prevalence of
vancomycin-resistant enterococci in the flora of healthy humans
decreased from 12% to 6%. In Germany, Klare et al. (1999) reported that
in the Saxony-Anhalt state, the prevalence of vancomycin-resistant
enterococci in the intestinal flora of healthy persons decreased from
12% in 1994, when avoparcin was still in use, to 6% in 1996 and 3% in
1997, after the antibiotic was banned. The authors also showed that by
the end of 1997, which marked almost 2 years from the avoparcin ban in
Germany, the prevalence of vancomycin-resistant enterococci in fresh and
frozen poultry meat from German producers decreased as compared to the
previous years, when the antibiotic was still in use.

Lauderdale et al. (2007) conducted a nationwide surveillance of 300
chicken farms in Taiwan, between 2000, when avoparcin was banned, and
2003, and revealed that the proportion of vancomycin-resistant
enterococci decreased from 13.7% to 3.7% and the proportion of E.
faecium from 3.4% to 0%, whereas resistance to several other antibiotics
tested remained similar between the 2 years.

O Resistant Bacteria in Food Products

Reports from several countries increasingly reveal the occasional
presence of resistant bacteria in commercial food products. Warren et
al. (2008) found CTX-M [beta]-lactamase genes, which represent a new
family of plasmid-mediated extended-spectrum [beta]-lactamases that
raise significant public health challenges worldwide (Bradford, 2001),
in imported chicken breasts, opening concerns that they could colonize
the intestinal tract of humans. Only one of 62 breasts reared in the
United Kingdom was resistant to fluoroquinolones, whereas 9 of 27
samples that were imported harbored the resistance gene. In a study that
analyzed meat products from Dutch supermarkets and butcher shops, van
Loo et al. (2007) found MRSA in 2 of 79 raw meat samples (2.5%). Kitai
et al. (2005) provided the first evidence of MRSA in commercial raw
chickens in Japan, when they detected the pathogen in 2 of 444
commercial samples collected from retail locations. MRSA was also
detected in 5 of 318 (1.6%) meat samples collected between November 2007
and March 2009 in La Rioja, Spain (Lozano et al., 2009). The first
survey to examine MRSA prevalence and characteristics in retail meat
products from the United States was conducted by Pu et al. (2009), who
randomly collected 120 samples from 30 grocery stores belonging to seven
supermarket chains in Baton Rouge, Louisiana, and isolated MRSA from one
beef and five pork samples.

Even though cooking destroys bacteria in food products, pathogens
pose a significant threat under several circumstances, such as during
the consumption of raw food. In addition, resistant pathogens may be
transmitted to processing-plant personnel or food handlers and colonize
their skin, nostrils, and gastrointestinal tract (de Boer et al., 2009).
Another place where contamination can occur is in the kitchen, during
food preparation, when cross-contamination of utensils, clothes,
fingers, and kitchen surfaces can facilitate pathogen dissemination.
Kusumaningrum et al. (2003) reported that the rate of S. aureus transfer
from artificially contaminated sponges to stainless steel surfaces
immediately after contamination was ~40%, and some bacteria remained on
sponges after the transfer, enabling additional cross-contamination.
Staphylococcus aureus was detected on dry stainless steel surfaces for
at least 96 hours when contamination was moderate or high, thus
illustrating its ability to survive and cross-contaminate other
surfaces. Scott and Bloomfield (1990) reported that S. aureus survives
on laminated surfaces and cloths, from which significant numbers can be
transferred to fingertips or inanimate surfaces even after a brief
contact, posing an infection hazard upon contact with food. A study
conducted in Ireland that examined the secondary dissemination of
pathogens from contaminated food in the kitchen described the
cross-contamination of multiple test sites with S. aureus, including
preparers' hands, refrigerator and oven handles, counter-tops, and
draining boards (Gorman et al., 2002).

O Ecosystems

For a long time, antibiotic resistance has been viewed as only a
clinical concern but, in reality, the topic is much more complex and
emerges as an environmental issue of global importance that is situated
at the crossroads of several disciplines (Cassone & Giordano, 2009).

To visualize the changes in the antibiotic resistance pattern of
environmental bacteria over time, Knapp et al. (2010) examined soil
samples retrieved from the soil archive in Alterra, at the Wageningen
University and Research Centre in The Netherlands, where samples have
been regularly deposited since 1879. The authors found that
microorganisms isolated from soil samples from 1940 through 2008
exhibited resistance to all classes of antibiotics that were tested, and
the current abundance of some resistance genes is more than 15x higher
than it was in the 1970s. This analysis identified some extended
spectrum [beta]-lactamase genes from periods that preceded their
emergence in the clinic, which points toward the potential origin of
these resistance determinants in soil bacteria (Knapp et al., 2010;
Wright, 2010).

Antibiotic-resistant bacteria were found in some of the most remote
locations on the planet. During a 2005 expedition organized by the
Swedish Polar Research Secretariat, Sjolund et al. (2008) examined 97
birds from three Arctic locations--northeastern Siberia, Point Barrow
(Alaska), and northern Greenland--and found eight antibiotic-resistant
strains, four of which were resistant to four or more antibiotics. Other
studies found antibiotic-resistant bacteria in remote human populations
that were not exposed to antibiotics. In September 1999, Bartoloni et
al. (2004) examined a very remote rural community of Guaranl Indians in
Bolivia, residing at ~1700 m altitude, having minimal exchanges with the
outside world and minimal antibiotic consumption, and found that >67%
harbored E. coli strains that were resistant to at least one antibiotic.
Subsequently, the authors studied a very remote population from the
Peruvian Amazonas, with minimal exposure to antibiotics and limited
exchange with the outside world, and found some commensal E. coli
strains that showed high levels of acquired resistance to some of the
oldest antibiotics (Bartoloni et al., 2009).

Rhodes et al. (2000) provide direct evidence that a specific type
of tetracycline-encoding plasmids, previously associated only with fish
farms, have disseminated between Aeromonas species and E. coli and
between aquaculture systems and human environments in at least four
countries: Norway, England, Germany, and Scotland. The authors suggested
that instead of thinking separately about the two compartments of the
environment, the fish farm and the hospital, we should instead envision
them as one interactive compartment in which the free exchange of
genetic information can occur.

To illustrate the dynamics of antibiotic resistance among
ecosystems, Baquero et al. (2008) described four main "genetic
reactors" where antibiotic resistance develops: the human and
animal microbes, which involve >500 species that are exposed to
prophylactic and therapeutic antibiotics; hospitals, long-term care
facilities, farms, and other locations where crowding exposes
individuals to bacterial infections; biological residues such as the
products of wastewater and sewage treatment plants that originate in the
secondary reactor, and where bacteria can mix and recombine; and the
soil and ground water, where bacteria can mix and interact with other
organisms.

The bacterial world is still filled with mysteries, and it is safe
to assume that we are still mostly ignorant about the ecology of
microorganisms on Earth. As Stephen Jay Gould pointed out, we live in
the "Age of Bacteria," in which microorganisms, the
"stayers and keepers" of life's history, exist in
tremendous numbers and varieties and populate "every place suitable
for the existence of life," from glaciers to hot springs (Gould,
1997). The number of bacterial cells that colonize our bodies exceeds
that of our own cells by a factor of 10 (Blaser & Falkow, 2009).
Studying bacteria and their interaction with other biological organisms
becomes even more challenging in light of the fact that most bacteria do
not grow under laboratory conditions, which historically represented the
first step toward isolating and characterizing them. Lewis (2007)
pointed out that >99% of microorganisms in nature cannot be cultured
and, thus, cannot be characterized, while >99.9% of the professionals
work on those microorganisms that can be cultured. This leads to a gap
in our understanding of the microorganisms that inhabit our planet and
interact with us and with other components of the various ecosystems.
The abundance of previously unexplored bacterial populations recently
started to emerge from metagenomic approaches, which involve the direct
isolation and examination of DNA from a sample, without the need to
culture the microorganisms. This ensures that DNA from bacteria that
would not have grown in culture is also present in the sample and can be
examined and characterized. By using a metagenomics approach, Venter et
al. (2004) found, in surface water samples collected from the Sargasso
Sea in Bermuda, an estimated 148 previously unknown bacterial
phylotypes, illustrating the diversity of the microbial world that this
powerful approach can unveil.

O What Can We Do?

Despite the initial success of antimicrobial treatments, infectious
diseases at the beginning of the 21st century remained the third leading
cause of death in the United States and the second leading cause of
death worldwide (Spellberg et al., 2004). With costs between $400 and
$800 million required for the research and development associated with
one pharmaceutical compound (DiMassa et al., 2003), the number of new
antibiotics approved by the Food and Drug Administration has seen a
decrease, from an average of 2.9 drugs annually in the 1960s, to 2.2 in
the 1990s, to 1.6 annually since 2000 (Powers, 2004). At the same time,
antibiotic resistance has been on the rise, and implementing measures
for the judicious use of antibiotics becomes a more urgent task than
ever.

Goossens et al. (2005) examined outpatient antibiotic use in 26
European countries between 1997 and 2002 and showed that while
antibiotic use varied widely among participating countries, resistance
rates were proportional with the amount of antibiotics being used.
Another study that involved 11 European countries found a linear
relationship between macrolide and [beta]-lactam antibiotic use and the
prevalence of penicillin-resistant S. pneumoniae isolates (Bronzwaer et
al., 2002).

One of the most important factors contributing to antimicrobial
resistance is the over-prescription of antibiotics and their
inappropriate prescription for conditions that do not respond to
antibiotics, such as viral infections. A survey conducted in 1992 that
included ~28,000 visits at >1500 office-based physicians'
offices found that ~12 million antibiotic prescriptions, or 21% of the
total antibiotic prescriptions for adults, were made for colds,
upper-respiratory-tract infections, and bronchitis, conditions that
normally do not benefit from antibiotics (Gonzales et al., 1997).
Similarly, a survey of >530 pediatric office visits found 6.5 million
antibiotic prescriptions, or 12% of the prescriptions written for
children in 1992, were for conditions that do not benefit from
antibiotics (Nyquist et al., 1998).

It was estimated that eliminating up to 50% of the human antibiotic
use, and up to 80% of the veterinary antibiotic use, could be possible
without major consequences (Dryden et al., 2009). Reducing antibiotic
use is one of the most promising interventions to prevent the emergence
of resistant isolates. In 1970, Price and Sleigh (1970) reported that in
a neurosurgery intensive-care ward where Klebsiella aerogenes had become
epidemic and had caused several infections between 1968 and 1969, the
number of infections, including those caused by other pathogens,
decreased after prophylactic and therapeutic antibiotics were
discontinued. In Iceland, Kristinsson (1999) showed that a reduction in
antibiotic use significantly reduced the number of resistant bacteria.

The risk of infection with a resistant microorganism depends not
only on individual antibiotic use but also on the presence of resistant
strains in the community. Walson et al. (2001) examined individuals from
three villages in Nepal and found that antibiotic use in the community
contributes to the presence of resistant bacteria in a person's
fecal flora to a larger extent than the person's individual
antibiotic consumption. This suggested that in addition to the
individual consumption of antibiotics, an individual's exposure to
others in the community who harbor resistant bacteria has an even
greater effect.

Antibiotic resistance takes on new dimensions in the current era of
global mobility, when any remote part of the planet can be reached
within hours, enabling the transcontinental and intercontinental spread
of resistant pathogens. This is vividly illustrated by
multidrug-resistant S. pneumoniae isolates that were collected in a
Cleveland daycare and shown, by molecular analyses, to be identical to a
strain previously characterized in Spain (Munoz et al., 1991).

The importance of good hygiene practices cannot be overstated.
Rahuma et al. (2005) examined 150 houseflies collected from streets, the
hospital, and an abattoir in the city of Misurata, Libya, and showed
that they are competent vectors enabling the transfer of
antibiotic-resistant microorganisms among communities. Elgderi et al.
(2006) collected 403 cockroaches from hospitals and households
surrounding hospitals in Tripoli, Libya, and, among the ones isolated
from the hospital setting, the authors described some that were
resistant to six or more antibiotics.

No consensus exists on how best to address the increasing threat of
antibiotic resistance. Some of the most frequent, and sometimes
conflicting, recommendations propose to reduce the number of
prescriptions, limit antimicrobial use in hospitals, decrease the use of
prophylactic antibiotics, rotate different classes of antibiotics in
time, reduce antibiotic use in agriculture and aquaculture, and
implement measures to control pathogen transmission among ecosystems
(Lipsitch & Samore, 2002; Bal et al., 2010). Most importantly, there
is an urgent need to implement educational activities that raise
awareness about the consequences of medical and nonmedical antibiotic
use, in an attempt to prevent, as Gould (2009) wrote, an
"ecological disaster of unknown consequence."

Gorman, R., Bloomfield, S. & Adley, C.C. (2002). A study of
cross-contamination of food-borne pathogens in the domestic kitchen in
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Subtherapeutic Use of Penicillin or Tetracyclines in Animal Feed.
Committee on Human Risk Assessment of Using Subtherapeutic Antibiotics
in Animal Feeds, Institute of Medicine, Division of Health Promotion and
Disease Prevention. Washington, DC: National Academy Press.

Smith, D.L., Harris, A.D., Johnson, J.A., Silbergeld, E.K. &
Morris, J.G., Jr. (2002). Animal antibiotic use has an early but
important impact on the emergence of antibiotic resistance in human
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